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Sect 38 rear window

mc607

mc607

Well Known Member
So my plan for the rear window is to use a modified #40 drill bit, I’ve ordered a #36 and #27 tapered drill reamer and will be picking up a new, good quality 6-32 tap tomorrow. I plan on using a newer Cleveland 3 flute countersink and swizel stick to do my countersinks. Will have the canopy as warm as I can get it, likely wait for a hot day like 25c minimum. I can borrow a cutting disc which came from Vans for a 10 build.

The only thing I didn’t get from my VAF research was how to polish the countersinks. Is this necessary if I’m using a countersink bit? If so, what do you use, what grit finish? Any pictures would be appreciated. Thanks in advance,
Todd
 
Van's has information on Drilling plexiglass in Section 5:

DRILLING
Special Plexiglas/Lexan drills are available from tool suppliers. Van's recommends two such tools; Diamond Dust drills and Unibits.
Clamping a piece of wood behind the acrylic material and drilling through into the wood can eliminate chipping on the backside of
drilled holes. Start drilling the warmed acrylic with slow speeds and light pressure. Increase speed and pressure as you progress.
As the drill bit starts to go through the canopy, reduce speed and pressure so that the drill bit penetrates the opposite surface
slowly. It is important to deburr both sides of the hole lightly with a machine countersink. No hole should have a sharp corner.
Practice drilling holes in scrap pieces until you are familiar and confident with the process. It is worth remembering that excessive
heat caused by machining and drilling may alter the acrylic's properties which may in turn allow even approved products to
negatively affect the material. For this reason it is important to cut slowly and avoid overheating the acrylic. To enlarge holes the
use of a step-drill (Unibit) or reamer is recommended.
CAUTION: DO NOT use a regular twist drill! A twist drill tends to fracture the acrylic due to its tip design. Using a regular
twist or plexi drill to enlarge a pre-drilled hole is not recommended and will practically guarantee a cracked canopy as a
result
Click to expand...

Don't try to "modify" a regular twist drill. Getting the proper zero rake angle on the flutes of a small #40 bit is next to impossible. The rake angle of zero degrees is super important. Any rake angle above that will cause the bit to grab and pull into the plastic and this will cause a crack. The zero rake angle makes the bit scrape its way down into the plastic. A unibit does this.
Also, don't try to tap the plexiglass. Only tap the underlying aluminum structure. The hole in the plexiglass should be enlarged so that the screw threads are well away from the plastic. Gently tighten the screws, then back them off a turn to allow for thermal expansion. Plexiglass will expand much more than the underlying aluminum.
 
bruceh said:
The hole in the plexiglass should be enlarged so that the screw threads are well away from the plastic.
Click to expand...

How large did you drill the plexiglass? And, I would have presumed a straight-flute, non-tapered reamer. Is that not correct?

Finally, did you oversize the canopy holes the same?
 
PermaGrit makes a great countersink with a #40 pilot, which is easy to center in the #27 holes in the window because it does not chatter. Available from Wicks (sometimes) or from PG in England. Originally developed for Lancairs, according to PG's website.
 
I dont know others experience, but the normal countersinks do a great job on plexi with no tendency to crack. Just go slow.
 
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